The present application relates to the field of differential signal transmission and, more particularly, to circuits that shift the common mode level of a differential signal without changing its differential amplitude.
Differential signals are a vital part of modern communication. In a differential signal, the information is not a function of the absolute voltage level on a conductor, but rather the difference of the voltages on a pair of conductors. Differential signals have several advantages over single-ended signals, which travel over a single conductor. Among these advantages are tolerance to outside electrical interference, lower electromagnetic emissions, ability to travel longer distances at higher speeds. Common examples of differential signaling include the local twisted wire pair used to connect telephones, Universal Serial Bus (USB) used in personal computers, and T10/100 connections between computers and routers, switches or modems. The receiver of a differential signal electrically subtracts, or finds the difference of, the value of one conductor from the other to decode the signal.
The noise tolerance features of differential signals are especially useful in high precision transmission where information is in analog form. A typical digital or binary signal might vary three volts to represent a single bit or On/Off piece of information. An analog signal might use the same three-volt range to convey many more (e.g., one thousand) possible different levels. This is accomplished by dividing the three-volt range into one thousand parts or 0.003 volts between adjacent levels. On a single conductor, this small voltage difference is easily corrupted by outside electrical noise sources, such as lightning, power supply noise, the switching on and off of nearby electrical loads, and of particular relevance in automotive applications, Schaffner pulses. This type of electrical noise tends to affect both conductors of a differential pair equally. Because in a differential signal, the signal is the voltage or current difference between conductors, this noise gets subtracted out at the receiver. This gives the differential circuit the ability to tolerate noise even while using small differential voltages.
Typical differential signals are not symmetrical about zero volts but rather vary between two levels, such as 9 and 10 volts. In this example, the average or common mode voltage is 9.5 volts. Theoretically, the common mode voltage is unimportant because the voltage difference is the important carrier of the information. As a practical matter, however, the common mode voltage is often significant because various receiving circuits can only tolerate a limited range of common mode voltage. A family of circuits called differential level shifters adjusts the common mode voltage of a differential signal without corrupting the information carried in the difference of the signal conductors.
Newly developed integrated circuit processes tend to use lower voltages than their predecessors. This may imply that only a lower common mode voltage is tolerable. Additionally, various standards may dictate or allow different, incompatible common mode voltages. Also, a newer, lower voltage integrated circuit process may need to receive a differential signal from an older, higher voltage, differential standard. In all cases, a differential level shifter is required.
Currently, differential level shifters are typically made on integrated circuits or from operational amplifiers and discrete components. It is important that the differential signal not be distorted in the process of shifting the common mode level. If the resistances at the two inputs of a differential receiver differ for example, the differential signal will not be preserved. This can result in inaccuracy and corruption of the signal. To avoid this problem, many differential level shifters use selected, matched, or trimmed components.
As the resolution of the differential signal gets finer and finer, the receiver components must become more precise. For example, a differential signal that encodes 100 levels into 1.0 volt must have precision to at least 0.01 volts or 1%. A signal that encodes 1,000 levels into 1.0 volt must have a precision of 0.001 volt or 0.1%. Because such precision is not common to integrated circuit processes, automated testers perform trimming or adjusting of fabricated integrated circuits. Precision integrated circuits such as differential level shifters may have built-in adjusting and trimming circuits to meet the required accuracy. Automated testers measure the as-built accuracy of the device and then trim various components by way of laser, mechanical, or chemical etching or the selective connection of auxiliary trim components by means of fusible links or semiconductor switches.
Semiconductor switches are a popular trimming means. Trimming circuits can switch trim resistors into or out of a circuit to adjust an overall resistance value. For example, the tester performing the trimming can turn on or off various switches to add or subtract resistance from the resistor being adjusted. Other uses of trimming with switches can add or remove active elements from the circuit to change the overall gain. This technique allows the matching or trimming of currents in circuits such as current mirrors. When the trimming is complete, the tester can destructively blow fuses inside the circuit or program memory elements to make the switch selection permanent.
However, such switches have their own disadvantages. For example, a semiconductor switch is fabricated differently than its corresponding trim resistor. Consequently, the thermal coefficient of resistance, also called the temperature coefficient, is different between the resistor and the switch that controls it. If one resistor is trimmed to match another resistor at a particular temperature, they can drift out of match as the integrated circuit becomes hotter or colder. This thermal drift causes errors in the accuracy of the differential signal. This was less of a problem with earlier designs when the needed precision was not as great. Now, with demands for greater precision, the temperature coefficient of the switch is a greater concern.